A communication system typically comprises multiple signaling nodes, such as user terminals, base stations, routers, switches, links, and so on. The nodes transmit and/or receive signals over a communication medium such as copper wire, optical fiber, or the atmosphere in the case of a radio interface.
In general, the signaling function requires some sort of signal amplification, since the amplitude of a signal is generally attenuated during transmission between nodes. For example, signals transmitted over a radio link may be attenuated due to such factors as propagation loss and multipath fading. A signal amplifier is thus typically provided to compensate for the attenuation.
In particular, a power amplifier (PA) is used to amplify a signal before transmission over a radio interface. When operated near saturation, PA's behave nonlinearly, leading to unwanted distortion of the signal. Such distortion can include so-called amplitude-amplitude (AM-AM) distortion and amplitude-phase (AM-PM) distortion.
To suppress unwanted PA nonlinearity, techniques such as using a pre-distorter have been investigated. A pre-distorter, disposed before a PA in the signal path, acts on an input signal in such a way that the combined effect of the pre-distorter and the PA is linear and memoryless. The advantages of using a pre-distorter include reducing spurious emissions, as well as improving power efficiency and in-band signal processing accuracy.
Various pre-distortion techniques have been described in the prior art. Look-up table based digital pre-distortion entails measuring the non-linear characteristics of a PA and storing a “mirror image” of those characteristics in a look-up table. Alternatively, such “mirror image” characteristics may be pre-programmed into pre-distortion components operating directly at RF in a technique called “analog feed-forward.” Yet another pre-distortion technique is polynomial-based digital pre-distortion, which entails digitally pre-distorting a signal at baseband using polynomial basis functions. With appropriate feedback, time-varying PA characteristics can be optimally adjusted using the latter approach.
The present disclosure describes various novel apparatuses and methods for linearizing non-linear output signals that may be used either in conjunction with or to the exclusion of the prior art techniques described above.